how to tell what's in the fringe? (lazy fringe)

[Drew Adams]

How can I tell (in Emacs-Lisp code) whether there is something other than blank
space somewhere in a given fringe (left or right)? Is that even feasible?

Use case -

I turned off fringe display as soon as it became available in Emacs. I've never
taken to it (ugh).

However, it seems that some things are now available _only_ in the fringe - see
bug #3550, for instance. That's not great design, IMO, but so be it.

So my thought was to code up some lazy fringe: a fringe that would not show,
except when there is some non-blank glyph in it. But I don't know how to test
whether that condition is satisfied (e.g. in Lisp).

Is that even logical? Perhaps if a fringe is not shown then its content is empty
or undefined? I was thinking in terms of it having content that is simply not
shown, but maybe that's incorrect.

Suggestions? Would this be a useful addition to Emacs? If so, would someone
else, who might be more familiar with fringe, like to implement it? ;-)